Protect partition members within Operation classes (#759726)

The Operation classes contain partition objects which are copied by
value.  Need to replace these with pointers to Partition objects instead
and manage their lifetimes so that they can be used polymorphically.

First step is to protect the partition members partition_new,
partition_original, and for OperationCopy class only, partition_copied
within the Operation classes and provide accessor methods.

get_partition_new() and get_partition_original() accessors are
implemented in the Operation base class so all derived classes get an
implementation.  get_partition_new() is also virtual so that
OperationCheck and OperationDelete can override the implementation and
assert that they don't use partition_new.  get_partition_copied() is
provided for the OperationCopy class only so can only be accessed via an
OperationCopy type variable.

Bug 759726 - Implement Partition object polymorphism